Union firefighters march into the state Capitol during a protest against a budget bill in Madison, Wis., in February 2011. The Bureau of Labor Statistics said the percentage of American workers in labor unions fell to 11.3 percent last year, the lowest ratio in 97 years.

The long decline in the number of U.S. workers belonging to labor unions accelerated sharply last year, sending the unionization rate to its lowest level in close to a century.

The Bureau of Labor Statistics said the total number of union members fell by 400,000 last year, to 14.3 million, even though the nation’s overall employment rose by 2.4 million nationwide.

The percentage of workers in unions fell to 11.3 percent, down from 11.8 percent in 2011, the bureau found in its annual report on union membership. That brought unionization to its lowest level since 1916, when it was 11.2 percent, according to a study by Rutgers economists Leo Troy and Neil Sheflin.

There were several reasons for the steep one-year decline, according to labor experts. Among the factors were new laws that rolled back the power of unions in Wisconsin, Indiana and other states, the continued expansion by manufacturers like Boeing and Volkswagen in nonunion states and the growth of sectors like retail and restaurants, where unions have little presence.

“These numbers are very discouraging for labor unions,” said Gary N. Chaison, a professor of industrial relations at Clark University in Worcester, Mass. “It’s a time for unions to stop being clever about excuses for why membership is declining, and it’s time to figure out how to devise appeals to the workers out there.”

Labor unions have boasted of their successes in helping re-elect President Barack Obama and in helping Democrats pick up seats in Congress.

But the figures announced by the bureau point to grave problems for organized labor’s future. The portion of private sector workers in unions fell to just 6.6 percent last year, from 6.9 percent in 2011, causing some labor experts to question whether private sector unions were sinking toward irrelevance. Private sector union membership peaked at around 35 percent in the 1950s.

The report showed particular drops in union membership in two groups where unions have long been strong: local government employees and manufacturing workers.

Union membership showed sharp drops in Wisconsin, which passed a law in 2011 curbing the collective bargaining rights of many public employees, and in Indiana, which enacted a right-to-work law last February that might have prompted many workers to drop their union membership.

Such laws bar requiring employees at unionized workplaces from being made to pay union dues or fees. The bureau’s report showed that union membership fell by 13 percent last year in Wisconsin and by 18 percent in Indiana — both unusually large drops for a single year.

Barry T. Hirsch, a labor economist at Georgia State University, said an analysis he conducted found that the number of government employees in Wisconsin belonging to a union slid by 48,000 last year, to 139,000 from 187,000, as many public sector workers evidently decided to quit their unions after the Republican-led Legislature stripped them of most of their bargaining rights.

Speaking about the nation as a whole, Hirsch said, “I am really surprised that the drop in unionization was as large as it is in a single year, and it was particularly big in the public sector. It does seem you are seeing reductions in some of the states that you might expect.”

For instance, in Indiana, where the right-to-work law took effect last March, unionization dropped to 9.1 percent from 11.3 percent in 2011. Michigan enacted a similar law last month.

The bureau said public sector union membership — long a labor stronghold — fell to 35.9 percent in 2012, from 37 percent the previous year. The number of government workers in unions fell by 234,000, as many teachers, police officers and others lost their jobs. There were 7.3 million public employees in unions, compared with 7 million private sector workers.

William Spriggs, the AFL-CIO’s chief economist, took a more upbeat approach to the report, noting that the bureau had found increased union membership in California, Georgia, North Carolina, Oklahoma and Texas.

“It’s not a simple story that we don’t have our act together,” Spriggs said. “I would be more concerned if union membership was down among Latinos and Asian-Americans, because that’s a growing demographic, but it’s up.”

He acknowledged that unions were doing poorly in manufacturing, retail and elsewhere in the private sector, which has been adding jobs even as union membership has continued a slide that has lasted for decades.

“Our labor laws do not favor unions organizing,” Spriggs said. “It would be one thing to say we’re bellyaching, but the Republican Party is really being vindictive against unions, and employers campaign very hard against workers unionizing.”

But Chaison said now would seem a good time for unions to attract workers.

“Workers should be looking to unions because of job insecurity and stagnant wages, but they’re not.”

In recent months, there has been an uptick in labor militancy as evidenced by recent protests at Wal-Marts across the nation and the one-day strike by fast-food workers in New York City in November. Both of those actions against nonunion employers protested what workers said were low wages and meager benefits.

Union officials acknowledge that it is often hard to persuade a majority of employees at a big-box store or other workplaces to vote to unionize.

Glenn Spencer, vice president of the Workforce Freedom Initiative of the U.S. Chamber of Commerce, said Wednesday’s report “has some alarm bells ringing at union headquarters across Washington.”

With workers no longer spending their entire career at one employer and often switching jobs, he said, workers no longer felt as attracted to unions.

“Unions have fundamentally had a hard time conveying to workers what their value proposition is, how they’re really going to make workers’ lives better,” Spencer said. “And if you look at union contracts and their rigid work rules, there is no incentive for employers to embrace unions either.”

According to the report, North Carolina has the lowest unionization rate, 2.9 percent, followed by Arkansas, at 3.2 percent. New York had the highest rate, 23.2 percent, with Alaska second, at 22.4 percent.

The bureau said that among full-time workers, union members had median weekly earnings of $943 last year (about $49,000 annually), compared with $742 (about $38,600 annually), for comparable nonunion workers.